7/17: 10:00AM - 1PM, 231 TH (Trowbridge Hall)

Title: Ordovician trilobite communities: End-Ordovician mass extinction beta diversity collapse, habitat occupancy dynamics, and insights from the Eleanor River Formation (Floian) fauna of Canada

Abstract: Trilobites (extinct marine arthropods) reached peak diversity during the Ordovician, when the group was divided into the Ibex and Whiterock evolutionary faunas. During the Early to early-Middle Ordovician, species of these faunas occupied habitats together along the depth gradient partitioned into four distinct, generally depth-restricted biofacies. Species richness within habitats (alpha diversity) was consistent throughout the period and into the Silurian, despite the loss of over 50% of global richness (gamma diversity) at higher taxonomic levels during the end-Ordovician mass extinction (EOME). This work addresses three goals: 1) description of a species rich, shallow water, bathyurid biofacies trilobite fauna from the Eleanor River Formation (Floian, Blackhillsian) of arctic Canada; 2) determination of whether a collapse in between-habitat dissimilarity (beta diversity) contributed towards the maintenance of alpha diversity after the EOME; and 3) examination of habitat preferences of the Ibex and Whiterock Faunas at the onset of the radiation and whether those preferences changed by the EOME.

The trilobite fauna of the Eleanor River Formation on Ellesmere and Devon Islands is more diverse than recognized in previous studies. At least 17 species are present, of which only Ischyrotoma anataphra Fortey, 1979 is previously described. Neostrotactinus gen. nov. and Eleanoria gen. nov. are proposed to resolve issues pertaining to Strotactinus and “Jeffersonia”-group bathyurids, respectively. New species described include: two species of Bathyurina Poulsen, 1937, two species of Eleanoria gen. nov., one species of Petigurus Raymond, 1913, and two species of Neostrotactinus gen. nov. Additional species include at least three asaphids, four bathyurids, a cheirurid, and a dimeropygid. Isoteloides polaris Poulsen, 1927 and Petigurusgroenlandicus Poulsen, 1937 are illustrated and redescribed, respectively, clarifying misinterpretations of the species' distribution.

Additional trilobite collections were compiled from the literature, theses, and unpublished datasets to form a comprehensive Laurentian occurrence dataset spanning the Early Ordovician through the Wenlock. These data were used to examine evolutionary fauna turnover and EOME response. Two-way cluster analyses indicate that depth-distinct biofacies persisted into the Late Ordovician, but lineages of the Whiterock Fauna were pervasive habitat generalists. Whiterock expansion across habitat types contributed to reduced biofacies distinction, and eventual replacement of some depth-restricted Ibex Fauna taxa by the Katian. Distinct biofacies disappeared in the Llandovery, associated with a significant decrease in Jaccard dissimilarity, as taxa became more widespread across the depth gradient and depth specialization was reduced. A shallow-subtidal dalmanitid biofacies formed in the Wenlock, distinct from a high diversity biofacies in preferentially deep subtidal habitats, but depth-agnostic taxa persisted across both. Species richness within higher Linnean ranks also increased in the early Silurian. Thus, alpha diversity was likely maintained through a combination of beta diversity collapse and rapid diversification within existing higher-level clades.